1. Field of the Invention
The present invention relates to an active matrix substrate suitable for use in an active matrix liquid crystal display device and to a method of producing such an active matrix liquid crystal display device. The present invention also relates to a liquid crystal display device and electronic equipment using such an active matrix substrate. More particularly, the present invention relates to a technique of annealing a semiconductor film on a surface of a substrate so as to melt and crystalize it.
2. Description of Related Art
Semiconductor films such as polycrystalline silicon are widely used in thin film transistors (hereinafter referred to as TFTs) and solar cells. The performance of these semiconductor devices is strongly dependent on the characteristics of semiconductor films forming active regions of the semiconductor devices. In other words, if a high-quality semiconductor film is obtained, it is possible to achieve a semiconductor device having high performance corresponding to the high quality of the semiconductor film. For example, in a polysilicon thin-film transistor (TFT) used in a liquid crystal display or the like, the switching speed of the TFT becomes higher and thus its performance improves with the degree of the quality of the polysilicon film used. In the case of a solar cell, the energy conversion efficiency increases with the degree of the crystallization of a semiconductor film if the semiconductor film has a similar light absorption coefficient. Thus, it is very important in various fields of industries to obtain a semiconductor film having good crystal characteristics.
However, it is generally difficult to form a high-quality semiconductor film. Furthermore, it is required that the semiconductor film be formed under particular conditions in very limited ranges. In the case of a TFT, a transistor is fabricated using high-temperature processes whose maximum temperature is about 1000.degree. C. so that a polysilicon film has a relatively high carrier mobility. This causes a limitation that the substrate on which a semiconductor film or semiconductor device is to be formed should have high resistance to the high-temperature processes. To meet the above requirement, polysilicon TFTs are generally produced on an expensive small-sized quartz glass substrate. For the same reason, solar cells are usually produced using amorphous silicon.
In the case of an active matrix substrate for use in a liquid crystal display device, it is desired to employ a low-cost glass substrate as the substrate on which thin film transistors (hereinafter referred to as TFTs) are formed. If a low-cost glass substrate is employed, it is required that production of TFTs on the substrate should be performed at a low temperature. Of various silicon films required to form channel and other regions of a TFT, an amorphous silicon film can be formed using a low-temperature process. However, the drawback of a TFT formed of such an amorphous silicon film is its low carrier mobility.
One technique proposed to avoid the above problem is to irradiate an amorphous silicon film formed on a glass substrate with a laser beam while moving the substrate so that the amorphous silicon film is melted and crystallized thereby obtaining a TFT with a high carrier mobility. In general, the substrate is moved relative to the laser beam so that the substrate is irradiated with a laser pulse in an overlapped fashion thereby producing a polysilicon film over a large area. This technique makes it possible to realize a TFT having a carrier mobility 10 times or greater than that of amorphous silicon.
However, in this conventional laser crystallization technique, when a substrate is irradiated with a laser beam while moving the substrate, nonuniformity occurs in the polysilicon film due to a variation of the laser power from pulse to pulse and/or due to overlapping of irradiation. Furthermore, it is required that the same area is irradiated by the laser beam at least 10 times to obtain a polysilicon film with high quality. This makes is difficult to achieve high throughput in production.
In view of the above problems, it is an object of the present invention to provide an active matrix substrate including a uniformly crystallized semiconductor film formed on a substrate. It is another object of the present invention to provide a method of producing an active matrix substrate, whereby a semiconductor film formed on a substrate can be uniformly crystallized by means of irradiation of a laser beam while achieving high throughput.